Could you tell me what happens if you accidentally sleep in your contact lens overnight?

Usually nothing: Unless this happens with hard lenses, there is little likelihood that you will have a problem if you sleep overnight with a lens in. Hard lenses can prevent oxygen transmission and you might developed an erosive syndrome which will clear when you remove the lens for several days. As a general rule, if the eyes become red for any reason or the vision drops, then take out the lens until cleared.